Sometimes geochemists can learn something new from looking closely at something
old, or something big from looking at something small. New tools for measuring
geochemical compositions and structures of geological materials on microscopic
scales are yielding exciting advances in our understanding of the Earth.

Good examples are
petrographic, geochemical, and isotopic studies of detrital zircons in metamorphosed
sediments from the Yilgarn Craton of Western Australia (Wilde and others, Nature,
v. 409, p. 175-178; Mojszis and others, Nature, v. 409, p. 178-181). These
studies revealed not only the oldest ages of any known minerals yet found in terrestrial
rocks -- 4.3 to 4.4 billion years old -- but also evidence for widespread liquid
water at the Earth's surface.

Another example is the insight gained into Phanerozoic seawater chemistry from
examination of the compositions of individual primary fluid inclusions from marine
halite deposits (Lowenstein and others, Science, 294, 1084-1086). The observed
variations in fluid inclusion compositions with time are correlated with seafloor
spreading rates, volcanic events, global sea level, and the mineralogies of marine
limestones and evaporites.

Scientific exploration of the sea floor
led to the discovery of a new type of submarine hydrothermal system, 15 kilometers
off the axis of the Mid-Atlantic Ridge at 30 degrees north latitude near its intersection
with the Atlantis Fracture Zone (Kelley et al., Nature, v. 412, p.145).
This field of hydrothermal vents, dubbed the "Lost City," is on 1.5
milion-year-old crust. In discharging alkaline vent fluids at relatively low temperatures
of 40 to 75 degrees Celsius that form carbonate chimneys up to 60 meters tall,
its chemistry is distinct from all other known submarine hydrothermal systems.
The chimney pictured here stretches as high as 30 feet. The white, sinuous spine
is freshly deposited carbonate material. Photo courtesy of the University of Washington/Woods
Hole Oceanographic Institution

Speaking of seawater, scientific exploration of the sea floor led to the discovery
of a new type of submarine hydrothermal system, 15 kilometers off the axis of
the Mid-Atlantic Ridge at 30 degrees north latitude near its intersection with
the Atlantis Fracture Zone (Kelley and others, Nature, 412, 145-149). This
field of hydrothermal vents, dubbed the "Lost City", is on 1.5 million
year old crust. . In discharging alkaline vent fluids at relatively low temperatures
of 40 to 75 degrees Celsius that form carbonate chimneys up to 60 meters tall,
its chemistry is distinct from all other known submarine hydrothermal systems.
Researchers think its characteristic fluid chemistry and heat content result from
exothermic serpentinization reactions, whereby the mineral olivine reacts with
seawater to form the mineral serpentine. The vent system supports dense microbial
communities including anaerobic thermophiles.

Not only do these seafloor hydrothermal systems support dense microbial communities,
so do mineral surfaces in laboratory beakers. As we noted in our geochemistry
highlights review last year (Geotimes, July 2001), synchrotron X-ray methods
are making a big difference in our understanding of small things, and biofilms
on mineral surfaces are no exception. In a study comparing lead adsorption on
polished alumina substrates in the presence and absence of B. cepacia biofilms,
a group at the Stanford Synchrotron Radiation Laboratory used the long-period
X-ray standing wave method. They found that the biofilm did not reduce the chemical
activity of the alumina substrate to lead adsorption. The biofilm adsorbed significant
amount of lead at solution concentrations exceeding one micromolar (Templeton
and others, Proc. Natl. Acad. Sci., v. 98, p. 11897-11902). This indicates
that although the presence of biofilms at mineral surfaces may alter the geochemical
behavior of the surfaces, it does not altogether mask their intrinsic behavior
as commonly speculated.

Another fundamental study, by a group using synchrotron X-ray reflectivity methods
at the Advanced Photon Source, discovered static water density oscillations at
the interface between muscovite and water and attributed these to the presence
of two types of adsorbed water: one, possibly hydronium, substituting for the
potassium ion in the ditrigonal sites; and the other forming a hydrogen-bonded
network extending to at least three water molecules out from the basal oxygen
plane of the tetrahedral sheet at the muscovite surface (Cheng and others, Phys.
Rev. Letters, v. 87, p. 156103-1-156103-4).

Adsorption of ions and molecules (especially organic molecules) at mineral surfaces
continues to be investigated intensively using new tools because microscopic,
or "nanoscopic", processes at mineral-fluid interfaces are believed
to control many macroscopic geochemical phenomena. For example, a study of organic
carbon in black shale from the Cretaceous Western Interior Seaway of North America
showed that 85% of the organic matter is correlated with mineral surface area,
suggesting that adsorption of organic compounds to clay mineral surfaces controls
the preservation and burial of organic carbon (Kennedy and others, Science,
v. 295, p. 657-660). Macromolecular organic matter in black shales provides sustenance
for microorganisms cultured from weathering profiles, as revealed by radiocarbon
analysis of their membrane lipids (Petsch and others, Science, v. 292,
p. 1127-1131). Another interesting discovery about natural organic matter is the
widespread formation of semivolatile chlorinated organic compounds from inorganic
chloride in soils during humification of plant material that was revealed by X-ray
absorption spectroscopy (Myneni, Science, v. 295, p. 1039-1041).

And finally, I'll touch on a few highlights of the noteworthy progress being made
in the realm of stable and radiogenic isotope geochemistry. Alan Matthews and
others, as well as Clark Johnson and others, measured isotope fractionation between
ferrous and ferric iron species in aqueous solutions, with both groups finding
heavy isotope enrichments in ferric iron exceeding one per mil per mass unit (Mathews
and others, Earth Planet Sci. Letters, v. 192, 81-92; Johnson and others,
Earth Planet. Sci. Letters, v. 195, p. 141-153), consistent with theoretical predictions
(Schauble and others, Geochim. Cosmochim. Acta, v. 65, p. 2487-2497). These
data will facilitate interpretation of iron isotope data for natural systems (e.
g., Sharma and others, Earth Planet. Sci. Letters, v. 194, p. 39-51). A
kinetic isotope effect of 3.4 per mil accompanying microbial reduction of dissolved
hexavalent chromium to trivalent chromium was measured in laboratory culture experiments
and natural waters, an important finding for applications in the environmental
geochemistry of toxic heavy metals (Ellis and others, Science, v. 295,
p. 2060-2062). Jane Barling and others first measurements of the isotopic composition
in natural molybdenum from marine sediments and seawater were reported to be on
the order of several per mil (Earth Planet. Sci. Letters, v. 193, p. 447-457).
A revised decay constant for lutetium-176 may have significant implications for
studies of early Earth history (Scherer and others, Science, v. 293, p.
683-687).

Overall, geochemical research in the twenty-first century has gotten off to a
running start. Interdisciplinary collaborations and advances in measurement techniques
are opening rich opportunities for new discoveries and new insights into old problems.

Back to indexSturchio is a professor in the
Department of Earth and Environmental Sciences at the University of Illinois at
Chicago, and is editor of the Geochemical News. E-mail.